Field of the Invention
The present invention is generally directed toward a process combining enzymatic debranching, melting, and crystallization to produce spherulites from linear α-1,4-linked glucans (e.g., short-chain amylose, SCA) with controlled enzyme digestibility. The linear α-1,4-linked glucans can be obtained by debranching starch, heating the mixture followed by cooling and crystallization to form well-developed spherulites.
Description of the Prior Art
Spherulites are important structural features found in many polymers crystallized from a melt. Starch-based spherulites may be obtained by cooling the starch suspension preheated into a solution state without disturbance. The overall morphology of spherulites is dependent on starch sources, amylose content, and crystallization conditions such as heating temperature, concentration of starting materials, cooling rate, and crystallization temperature. High-amylose starches form spherical structures with birefringence more readily than normal and waxy starches. Spherulites may form over a wide range of cooling rates (1-250° C./min) provided that amylose solution (10 to 20%, w/w) was preheated to greater than 170° C. Spherulitic crystals display dimensions and structural characteristics consistent with the hilum and core region of native granules, thus have been proposed as a model for starch granule initiation in vivo; however, in those studies, amylose was isolated from granular starch by an aqueous leaching process, and the spherulites were prepared in a differential scanning calorimetry (DSC) pan. A small sample size was used and was not enough for conducting digestion study.
Spherulitic crystallization from acid hydrolyzed potato starch solution has been documented as well. Helbert et al., Morphological and structural features of amylose spherocrystals of A-type. Intern. J. Biol. Macromol. 1993, 15, 183-187, prepared spherulites by mixing ethanol with hot aqueous solutions of low molecular weight amylose followed by slow cooling to 4° C. The precipitates had a diameter of the order of 10 μm and exhibited an A-type X-ray diffraction pattern. In contrast, spherulites with a B-type polymorph and a dimension of 10-15 μm were produced by direct cooling 5-20% w/w aqueous acid hydrolyzed potato starch solution to 2° C. Ring et al., Spherulitic crystallization of short chain amylose. Intern. J. Biol. Macromol. 1987, 9, 158-160. Because A- and B-amylose spherulites mimic both granular morphology and the crystalline types of native starches, they were used as model systems to study the enzymatic hydrolysis of starch crystallites. Planchot et al., Enzymatic hydrolysis of α-glucan crystallites. Carbohydr. Res. 1997, 298, 319-326, and Williamson et al., Hydrolysis of A-type and B-type crystalline polymorphs of starch by alpha-amylase, beta-amylase and glucoamylase-1. Carbohydr. Polym. 1992, 18, 179-187. The materials those authors used to prepare spherulites were obtained by extensive acid hydrolysis of native starches with hydrochloric acid. Significant loss of starch occurred during acid treatment since washing was needed to remove water-soluble products and recover acid resistant products. The product also presumably still contained branched points.